Composition for synthetic resin magnet and formed resin magnet

ABSTRACT

A composition for synthetic resin magnets (such as magnet rollers for electrophotography process) which is composed of a resin binder and a magnetic powder mixed and dispersed therein. The resin compound has improved melt flow rate because the binder contains a thermoplastic and an aliphatic polyamide. Owing to its improved melt flow rate, the resin compound can be incorporated with a large amount of magnetic powder without deterioration in moldability. Thus the resulting plastic magnet has a strong magnetic force. In addition, the composition for synthetic resin magnets may be incorporated with a hindered phenol antioxidant, so that it is molded at a higher temperature (120-180° C.) than usual to achieve the same object as mentioned above.

TECHNICAL FIELD

The present invention relates to a composition for synthetic resinmagnets and a molded formed resin magnet. The resin compound is composedof a resin binder and a magnetic powder mixed and dispersed therein. Theresin compound is suitable as a raw material to be molded into plasticmagnets, such as magnet rollers, which are incorporated intoelectrophotographic machines and electrostatic recording machines.

BACKGROUND ART

It has been known that those copying machines and printers which arebased on the principle of electrophotography or electrostatic recordingemploy a developing roller to visualize an electrostatic latent imageformed on a latent image supporter, such as sensitive drum. Thedeveloping roller consists of a rotating sleeve and a magnet rollerplaced therein. The magnetic roller is a molded plastic magnet and has aprescribed magnetized pattern. The developing roller attracts a magneticdeveloper (or toner) according to the magnetized pattern and transfersit to the latent image supporter by so-called jumping.

The above-mentioned magnetic roller is formed from a resin compoundcomposed of a thermoplastic binder and a magnetic powder mixedtherewith. Forming is accomplished by injection molding or extrusionmolding in a magnetic field such that a desired magnetic pattern is madeon the roller surface.

The recent advance in electrophotographic technology requires a morecomplex magnetic pattern on the magnet roller than before. One way tomeet this requirement is to form a plurality of magnet pieces from theabove-mentioned resin compound, which are magnetized according to thedesired magnetic pattern, and arrange them on the surface of the shaft.

Such a magnet roller is conventionally prepared from a resin compoundcomposed of a resin binder and a magnetic powder of ferrite or rareearth alloy dispersed therein. The resin binder is usually polyamideresin, such as polyamide-6 and polyamide-12, or polypropylene.

Nowadays, the magnet roller is required to have a stronger magneticforce as OA machines become higher precision, more sophisticated andneed speedier operation. In other fields, too, there is an increasingdemand for plastic magnets having a stronger magnetic force than before.

A conceivable way to meet this requirement is to increase the mount offerrite magnetic powder to be incorporated into the resin compound fromwhich magnet rollers are molded. The disadvantage of increasing theamount of ferrite magnetic powder is that the resulting resin compoundis extremely poor in melt flow properties and moldability. Formed partsobtained from such a resin compound are uneven in magnetic force andpoor in dimensional accuracy. Therefore, the amount of magnetic powderis limited as a matter of course, and this prevents incorporation withas much magnetic powder as necessary to meet the requirement for strongmagnetic force.

The object of increasing magnetic force is achieved by replacing theferrite magnetic powder with rare earth magnetic powder. However, thelatter still poses a problem with poor melt flow properties if it isused in an amount enough for the desired magnetic force. A highly filledresin compound easily suffers short shot when molded into small magnetrollers.

DISCLOSURE OF THE INVENTION

The present invention was completed in view of the foregoing. It is anobject of the present invention to provide a composition for syntheticresin magnets and a formed resin magnet, said resin compound having goodmelt flow properties and moldability despite its high content ofmagnetic powder filled therein and being capable of molding into aplastic magnet having a strong magnetic force.

In order to achieve the above-mentioned object, the present inventorscarried out a series of researches which led to the finding that it ispossible to effectively improve the melt flow rate of the compositionfor synthetic resin magnets if, when the resin compound is prepared froma thermoplastic resin binder and a magnetic powder, the resin binder isincorporated with an aliphatic polyamide represented by the formula (1)below. The resin compound obtained in this manner keeps good melt flowproperties even though it is incorporated with a large amount ofmagnetic powder so that the formed part has a strong magnetic force. Inthis way it is possible to produce desired plastic magnets withoutproblems with poor melt flow properties and poor moldability.

(where R₁ denotes HOOC(CH₂)_(n)COOH (n=7 or 8), C_(m) denotes a diamineresidue chain (m=2-20), C_(n) denotes a dimer acid residue chain(n=20-48), a is an integer of 1-50, b is an integer of 1-50, and x is aninteger of 1 to 50.)

The first aspect of the present invention resides in a composition forsynthetic resin magnets which is composed of a resin binder and amagnetic powder mixed and dispersed therein, wherein said resin bindercomprising a thermoplastic resin as a major constituent and an aliphaticpolyamide, more specifically, the one which is represented by theformula (1) above. It also covers a formed resin magnet which isobtained from said resin compound.

Moreover, in order to achieve the above-mentioned object, the presentinventors carried out a series of researches, with emphasis placed onthe additive and molding conditions, which led to the finding that acomposition for synthetic resin magnets which is composed of a resinbinder and a magnetic powder mixed and dispersed therein has improvedmelt flow properties and gives a formed resin magnet with a highdimensional accuracy and good magnetizing performance with reducedvariation in surface magnetic force, if it is incorporated with aprescribed amount of hindered phenol antioxidant and the resulting resincompound is molded at 120-180° C.

Thus, the second aspect of the present invention resides in a formedresin magnet which is molded in a desired shape at 120-180° C. from acomposition for synthetic resin magnets which is composed of a resinbinder, a magnetic powder, and a hindered phenol antioxidant.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the change in torque with time that takesplace when the composition for synthetic resin magnets is prepared inExample 1 and Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description is given below of the first and second aspects ofthe present invention.

[The First Aspect of the Invention]

The first aspect of the present invention is concerned with acomposition for synthetic resin magnets which is composed of a resinbinder and a magnetic powder mixed and dispersed therein, said resinbinder being composed of a major constituent of thermoplastic resin andan aliphatic polyamide.

The resin binder contains as a major constituent a thermoplastic resinwhich is one or more members selected from polyamide resin (polyamide-6,polyamide-12, etc.), polystyrene resin, polyethylene terephthalate resin(PET), polybutylene terephthalate resin (PBT), polyphenylene sulfideresin (PPS), ethylene-vinyl acetate copolymer resin (EVA),ethylene-ethyl acrylate resin (EEA), epoxy resin, ethylene-vinyl alcoholcopolymer resin (EVOH), polypropylene resin (PP), polyolefins (such aspolyethylene and polyethylene copolymer), and modified polyolefins(formed from polyolefins by introduction of reactive functional groupssuch as maleic anhydride group, carboxyl group, hydroxyl group, andglycidyl groups).

The thermoplastic resin as a major constituent is not specificallyrestricted in loadings. An adequate amount is 1-20 wt %, preferably 4-16wt %, of the total amount of the composition for synthetic resinmagnets. With an amount less than 1 wt %, the thermoplastic resin doesnot effectively contribute to improvement in melt flow properties eventhough an aliphatic polyamide (mentioned later) is added. Moreover, theresulting plastic magnet will be very brittle. On the other hand, withan amount more than 20 wt %, the thermoplastic resin accounts for alarger portion than the magnetic powder, and this makes it difficult toproduce a plastic magnet having a strong magnetic force.

The major constituent of thermoplastic resin is incorporated with analiphatic polyamide as mentioned above. This aliphatic polyamide is notspecifically restricted so long as it achieves the object of the presentinvention. The one represented by the formula (1) below is preferable.

(where R₁ denotes HOOC(CH₂)_(n)COOH (n=7 or 8), C_(m) denotes a diamineresidue chain (m=2-20), C_(n) denotes a dimer acid residue chain(n=20-48), a is an integer of 1-50, b is an integer of 1-50, and x is aninteger of 1 to 50.)

An additional comment is made below on the symbols in the formula (1)above. R₁ denotes a dicarboxylic acid represented by HOOC(CH₂)_(n)COOH,such as azelaic acid (n=7) and sebacic acid (n=8). The formula may havetwo kinds of blocks mixed together, each block containing azelaic acid(n=7) or sebacic acid (n=8). Cm denotes a diamine residue chain(m=2-20). It typically includes ethylenediamine,1,4-diaminobutanehexamethylenediamine, nonamethylenediamine,undecamethylenediamine, dodecamethylenediamine,2,2,4-trimethylhexamethylenediamine, bis-(4,4′-aminocyclohexyl)methane,and m-xylenediamine. C_(n) denotes a dimer acid residue chain (n=20-48).It typically includes dimmers of oleic acid, linoleic acid, and erucicacid. In the formula, a is an integer of 1-50, b is an integer of 1-50,and x is an integer of 1 to 50. Incidentally, the aliphatic polyamiderepresented by the formula (1) should preferably (although notmandatory) have a number-average molecular weight of 1000-65000, moredesirably 5000-25000.

The aliphatic polyamide represented by the formula (1) typicallyincludes “PA-30L”, “PA-30”, “PA-40L”, “PA-40”, “PA-30R”, “PA-30H”,“PA-50R”, “PA-50M”, “PA-60”, “PA-160”, and “PA-260”, which are availablefrom Fuji Kasei Kogyo Co., Ltd. Those aliphatic polyamides which are notrepresented by the formula (1) above may also be used; however, those ofelastomer type are excluded in the present invention.

The amount of the aliphatic polyamide varies without specificrestrictions depending on the kind of the major constituent resin andthe loadings of magnetic powder. It should preferably be 0.1-20 wt %,particularly 0.1-5 wt %, of the total amount of the composition forsynthetic resin magnets. With an amount less than 0.1 wt %, thealiphatic polyamide does not fully produce the effect of improving themelt flow properties. With an amount more than 20 wt %, the aliphaticpolyamide accounts for a large portion relative to magnetic powder,preventing the resulting plastic magnet from producing a sufficientmagnetic force.

The binder resin composed of the major constituent resin and thealiphatic polyamide is incorporated with a magnetic powder as mentionedabove. This magnetic powder may be any known one which has been used forconventional plastic magnets. It typically includes powder of ferritesuch as Sr ferrite and Ba ferrite, and powder of rare earth alloy suchas alnico alloy, Sm—Co alloy, Nd—Fe—B alloy, Sm—Fe—N alloy and Ce—Coalloy.

The magnetic powder used in the present invention is not specificallyrestricted in particle diameter. However, it should preferably be onewhich has an average particle diameter of 0.05-300 μm, particularly0.1-100 μm, so that it has good orientation and loading properties andit has no adverse effect on the melt flow properties of the compositionfor synthetic resin magnets.

The magnetic powder may undergo any known surface treatment with acoupling agent, such as silane coupling agent or titanate couplingagent, before incorporation into the composition for synthetic resinmagnets. The treated magnetic powder effectively contributes to the meltflow properties when it is incorporated in a large amount.

Preferred examples of silane coupling agent includeγ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane,N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane,ureidopropyltriethoxysilane, vinyltriethoxysilane,vinyltrimethoxysilane, vinyltris(β-methoxyethoxy)silane,γ-methacryloxy-propyltrimethoxysilane,γ-methacryloxypropyltriethoxysilane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane,γ-isocyanatepropyltriethoxysilane, methyltriethoxysilane, andmethyltrimethoxysilane. Of these examples,γ-amino-propyltriethoxysilane, γ-aminopropyltrimethoxysilane, andN-β-(aminoethyl)-γ-aminopropyltrimethoxysilane are preferable.

Preferred examples of titanate coupling agent includeisopropyl-bis(dioctylpyrophosphate) titanate,isopropyl-tri(N-aminoethyl-aminoethyl) titanate,isopropyl-triisostearoyl titanate, diisopropyl-bis(dioctylpryophosphate)titanate, etraisopropyl-bis(dioctylphosphite) titanate,tetraoctyl-bis(ditridecylphosphite) titanate,tetra(2,2-diallyloxymethyl-1-butyl)-bis-(ditridecyl)phosphite titante,bis(dioctylpyrophosphate)oxyacetate titanate, andbis(dioctylpyrophosphate)ethylene titanate. Of these example,isopropyl-bis(dioctylpyrophosphate) titanate is particularly desirable.

The amount of the magnetic powder may vary without specific restrictionsdepending on magnetic force required of the formed resin magnet. It isusually 80-99 wt % of the total amount of the composition for syntheticresin magnets. According to the present invention, the loadings ofmagnetic powder may exceed 90 wt % without adverse effect on the meltflow properties of the resin compound and the moldability of plasticmagnets with a strong magnetic force. The present invention produces itsremarkable effect in the case of high loadings with magnetic powder.However, the resin compound of the present invention permits uniformdispersion of magnetic powder even though the amount of loadings israther small, say, about 80-90 wt %.

According to the present invention, the composition for synthetic resinmagnets is composed of the major constituent resin, aliphatic polyamide,and magnetic powder as mentioned above. It should preferably (althoughnot mandatory) be additionally incorporated with an adequate amount ofantioxidant to protect the binder resin from deterioration due tooxidation. Any known antioxidant may be used without specificrestrictions. Its typical examples include those derived from hinderedphenol, hindered mine, and phosphorus.

The amount of antioxidant may vary without specific restrictionsdepending on the kind of antioxidant and binder resin. It shouldpreferably be 0.1-20 wt %, particularly 0.1-3 wt %, of the total amountof the composition for synthetic resin magnets.

According to the present invention, the composition for synthetic resinmagnets may optionally be incorporated with a dispersing agent,lubricant, and plasticizer in an adequate amount.

The dispersing agent includes phenol-based ones and amine-based ones.The lubricant includes waxes such as paraffin wax and microcrystalline,and fatty acids such as stearic acid and oleic acid, and metal saltsthereof such as calcium stearate and zinc stearate. The plasticizerincludes monoester and polyester plasticizers and epoxy plasticizers.

In addition, according to the present invention, the composition forsynthetic resin magnets may optionally be incorporated with areinforcing filler such as mica, whisker, talc, carbon fiber, and glassfiber, in an amount not harmful to the effect of the present invention.The formed resin magnet tends to be less rigid if it merely requires acomparatively weak magnetic force and hence the amount of magneticpowder therein is comparatively small. In this case it is desirable toadd a reinforcing filler, such as mica and whisker, to increaserigidity. Reinforcing fillers suitable for the present invention aremica and whisker. Examples of the whisker include non-oxide whiskersformed from silicon carbide or silicon nitride, metal oxide whiskersformed from any of ZnO, MgO, TiO₂, SnO₂, and Al₂O₃, and compound oxidewhiskers formed from potassium titanate, aluminum borate, and basicmagnesium sulfate. Of these examples, compound oxide whiskers aredesirable because of their good miscibility with resin.

The amount of the reinforcing filler is not specifically restricted;however, it is usually 1-50 wt %, preferably 5-20 wt %, of the totalamount of the composition for synthetic resin magnets. Incidentally, thecomposition for synthetic resin magnets may be incorporated with anyadditives other than the above-mentioned dispersing agent, lubricant,plasticizer, and filler, within the scope of the present invention.Examples of such additives include organotin stabilizers.

The following deals with the formed resin magnet pertaining to the firstaspect of the present invention, which is obtained from the compositionfor synthetic resin magnets.

The formed resin magnet pertaining to the first aspect of the presentinvention is obtained by molding from the above-mentioned compositionfor synthetic resin magnets. It is characterized by high dimensionalaccuracy, uniform magnetic force, and strong magnetic force. Thecomposition for synthetic resin magnets according to the presentinvention retains good melt flow properties even though it isincorporated with a large amount of magnetic powder. Therefore, itpermits incorporation with a large amount of magnetic powder necessaryfor a strong magnetic force and yet it readily flows without short-shotin mold cavities at the time of molding, permitting uniform dispersionand orientation of magnetic powder. The formed resin magnet thusobtained is suitable for high-performance magnet rollers with a strongmagnetic force.

The formed resin magnet can be readily obtained by molding thecomposition for synthetic resin magnets in molten state. Molding may beaccomplished by any of injection molding, extrusion molding, compressionmolding, etc. which is suitable for the desired plastic magnet. Ordinarymolding conditions may be employed according to the composition of theresin compound and the shape of the desired plastic magnet.

The first aspect of the present invention is characterized in that thecomposition for synthetic resin magnets is based on a binder resinincorporated with an aliphatic polyamide. Therefore, the resin compoundhas good melt flow properties and hence exhibits good moldability at thetime of injection molding, extrusion molding, compression molding, etc.This makes it possible to produce a plastic magnet with high loadings ofmagnetic powder for a strong magnetic force without adverse effect onmoldability.

[The Second Aspect of the Invention]

The second aspect of the present invention is concerned with a formedresin magnet which is obtained by molding into a desired shape at120-180° C. from a composition for synthetic resin magnets which iscomposed of a resin binder, magnetic powder, and hindered phenolantioxidant.

The resin binder in the resin compound is not specifically restricted;it may be the same thermoplastic resin as exemplified as the majorconstituent resin in the resin compound pertaining to the first aspectof the present invention. More than one thermoplastic resin may be usedalone or in combination. In the second aspect of the present invention,the thermoplastic resin may be any of polyamide-6, polyamide-12,polyamide-66, polyamide-11, and polyamide-46. Of these examples,polyamide-12 and polyamide-6 are particularly preferable.

The resin binder may be mixed with an aliphatic polyamide as in thefirst aspect of the present invention.

The resin binder may be incorporated with any magnetic powder which isnot specifically restricted. Examples of the magnetic powder includeferrite powder, alnico alloy powder, and rare earth alloy powder, whichwere exemplified above in the first aspect of the present invention. Inthe second aspect of the present invention, rare earth alloy powder ispreferable because of its strong magnetic force. Preferred examples ofthe rare earth alloy powder include Nd-based magnetic powder such asNd₂Fe₁₄B and Nd₁₂Fe₇₈Co₄B₆, and Sm-based magnetic powder such asSm₂Fe₁₇N₃. These magnetic powders may be used alone or in combinationwith one another. The magnetic powder is not specifically restricted inparticle diameter. However, it should preferably have an averageparticle diameter of 1-250 μm, particularly 20-50 μm, so that it hasgood orientation and loading properties and it has no adverse effect onthe melt flow properties of the composition for synthetic resin magnets.Also, the magnetic powder may be previously surface-treated with asilane coupling agent or the like in the same way as in the first aspectof the present invention.

In the composition for synthetic resin magnets from which the formedresin magnets is obtained according to the second aspect of the presentinvention, the mixing ratio of the resin binder and the magnetic powdervaries without specific restrictions depending on the strength ofmagnetic force required of the resulting formed resin magnet. Usually,the amount of magnetic powder is 70-95 wt % of the total amount of thecomposition for synthetic resin magnets (having a density of 2.5-6.0g/cm³). According to the present invention, the resin compound may beincorporated with more than 80 wt %, particularly 80-95 wt %, ofmagnetic powder (for the resin compound to have a density of 3.2-6.0g/cm³), without adverse effect on the melt flow properties, owing to theincorporation with a hindered phenol antioxidant (mentioned later) whichimproves the melt flow properties.

According to the present invention, the composition for synthetic resinmagnets is incorporated with a hindered phenol antioxidant. The hinderedphenol antioxidant is not specifically restricted; any commercial onescan be used. Typical examples are listed next.N,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hydrazine, “IRGANOXMD 1024”, from Ciba Specialty Chemicals K.K. Triethyleneglycolbis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], “IRGANOX 245,245FF, 245DWJ”, from Ciba Specialty Chemicals K.K. Pentaerythritoltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], “IRGANOX 1010,101OFP, 1010FF”, from Ciba Specialty Chemicals K.K. Thiodienebis[3-(3,5-t-butyl4-hydroxyphenyl)propionate], “IRGANOX 1035, 1035FF”,from Ciba Specialty Chemicals K.K.Octadecyl-3-(3,5-di-t-butyl-4hydroxyphenyl)propionate, “IRGANOX 1076,1076FF, 1076FD, 1076DWJ”, from Ciba Specialty Chemicals K.K.N,N′-hexane-1,6-diylbis[3,5-di-t-butyl-4-hydroxyphenylpropionamide],“IRGANOX 1098”, from Ciba Specialty Chemicals K.K. Ester ofbenzenepropanoic acid with 3,5-bis(1,1′-dimethylethyl)-4-hydroxyalkyl(C₇, C₉ side chains), “IRGANOX 1135”, from Ciba Specialty Chemicals K.K.2,4-dimethyl-6-(1-methylpentadienyl)phenol+IRGANOX 1076, “IRGANOX 1141”,from Ciba Specialty Chemicals K.K.Diethyl{[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl} phosphate,“IRGANOX 1222”, from Ciba Specialty Chemicals K.K. 3,3′,3″,5,5′,5″-hexa-t-butyl-a,a′,a″-(mesitylene-2,4,6-toluyl)tri-p-cresol,“IRGANOX 1330”, from Ciba Specialty Chemicals K.K. Calcium diethylbis{[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-methyl]phosphate}+polyethylenewax, “IRGANOX 1425WL”, from Ciba Specialty Chemicals K.K.4,6-bis(octylthiomethyl)-o-cresol, “IRGANOX 1520L”, from Ciba SpecialtyChemicals K.K. Hexamethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], “IRGANOX 259”, fromCiba-Specialty Chemicals K.K.1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H, 3H,5H)-trione, “IRGANOX 3114”, from Ciba Specialty Chemicals K.K.1,3,5-tris[(4-t-butyl-3-hydroxy-2,6-xylyl)methyl-1,3,5-triazine-2,4,6-(1H,3H, 5H)-trione], “IRGANOX 3790”, from Ciba Specialty Chemicals K.K.Reaction product of N-phenylbenzeneamine and 2,4,4-trimethylpentene,“IRGANOX 5057”, from Ciba Specialty Chemicals K.K.2,6-di-t-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol,“IRGANOX 565, 565DD”, from Ciba Specialty Chemicals K.K.Tris(2,4-di-t-butylphenyl)phosphite, “IRGANOX 168, 168FF”, from CibaSpecialty Chemicals K.K.

The amount of the hindered phenol antioxidant to be added is adequatelydetermined according to the kind of the magnetic powder and binderresin. It is usually 0.1-20 wt %, preferably 0.1-5 wt %.

As in the first aspect of the present invention, the composition forsynthetic resin magnets may optionally be incorporated with theabove-mentioned dispersing agent, lubricant, plasticizer, and areinforcing filler such as mica, whisker, talc, carbon fiber, and glassfiber, in an amount sufficient to disperse the magnetic powder.

The composition for synthetic resin magnets may be prepared in anymanner without specific restrictions. For example, one method forpreparation consists of mixing together the resin binder and magneticpowder and optional dispersing agent and filler in the usual way. Theresulting mixture is formed into pellets by melt-mixing. Thus there isobtained the desired resin compound in pellet form. Melt-mixing may beaccomplished in the usual way under ordinary conditions by using asingle-screw or twin-screw extruder or KCK extruder.

The formed resin magnet according to the present invention is obtainedby molding in a desired shape at 120-180° C. from the composition forsynthetic resin magnets which contains a hindered phenol antioxidant asmentioned above. Molding may be accomplished by injection molding,extrusion molding, compression molding, etc., with the mold temperaturekept at 120-180° C. Injection molding is preferable.

According to the second aspect of the invention, by synergistic effectof setting the molding temperature higher than 85-100° C. as usual andimproving the flow property of the hindered phenol antioxidant, theformed resin magnet is obtained in the condition that the melt flow rate(MFR) of the composition for synthetic resin magnets injected in themold cavity is accomplished to be high. Thus, the formed resin magnet,which has a high dimensional accuracy and a strong surface magneticforce with uniform distribution, can be obtained.

As mentioned above, the second aspect of the present invention offersthe advantage that the composition for synthetic resin magnets issuperior in melt flow properties because it contains, in addition to theresin binder and magnetic powder, a hindered phenol antioxidant whichimproves melt flow properties and it is molded at 120-180° C. Therefore,according to the second aspect of the present invention, it is possibleto provide a formed resin magnet which has a high dimensional accuracyand a uniform strong magnetic force.

The composition for synthetic resin magnets pertaining to the first andsecond aspects of the present invention will find various applicationswithout specific restrictions. Particularly preferred examples includemagnet rollers (and parts thereof) used in electrophotographic machinesand electrostatic recording machines. Such rollers are required to havea strong magnetic force and advanced magnetic performance. Such amagnetic roller usually consists of a roller proper (of plastic magnet)and shafts projecting from both ends thereof. In this case, the desiredmagnet roller may be formed around a metal shaft which has previouslybeen placed in the mold. Alternatively, the shafts may be moldedintegrally with the magnet roller proper.

In the case where sophisticated magnetic performance is required, thedesired roller may be formed by attaching to a metal shaft rod-shapedplastic magnets which have previously been formed from the resincompound pertaining to the first or second aspect of the presentinvention. In this case it is not always necessary to form all therod-shaped plastic magnets from the resin compound pertaining to thefirst or second aspect of the present invention. Instead, only thoseplastic magnets which need a particularly strong magnetic force may beformed from the resin compound pertaining to the first or second aspectof the present invention. Magnetization of the magnet roller may beaccomplished simultaneously with molding in a magnetic field formedaround the mold, or after molding by using any known magnetizingmachine.

EXAMPLE

The invention will be described in more detail with reference to thefollowing Examples and Comparative Examples, which are not intended torestrict the scope thereof.

Example 1

A magnetic powder having an average particle diameter of 100 μm wasprepared by crushing an Nd-based rare earth alloy having a compositionof Nd₁₂Fe₇₈Co₄B₆ (in atom wt %), “MQP-B” made by General Motors Inc. Themagnetic powder was surface-treated with a silane coupling agent,“A1100” from Nippon Unicar Co., Ltd.

The following components were mixed at 250° C. for 15 minutes with arotational speed of 50 rpm, by using “Labo Plastomill, Model 50C150” (60cm³ in capacity), made by Toyo Seiki Co., Ltd. Magnetic powder: 188 g(mentioned above), Nylon-12: 6.8 g, “P 3012 U” from Ube Industries.Ltd., Antioxidant: 3.5 g, “IRGANOX MD 1024” from Ciba SpecialtyChemicals K.K., Aliphatic polyamide: 1.7 g, “PA-30L” from Fuji KaseiCo., Ltd. Thus, there was obtained the composition for synthetic resinmagnets pertaining to the first aspect of the present invention. Duringmelt mixing, the melt was examined for change in torque values. Theresults are shown in FIG. 1. The resin compound with high torque valueshas a high melt viscosity and hence is poor in melt flow properties. Itis to be noted from FIG. 1 that the resin compound in Example 1 retainslow torque values (and hence low melt viscosity) during mixing for 15minutes.

The thus obtained composition for synthetic resin magnets was tested formelt flow rate (MFR) by using a melt indexer (made by Toyo Seiki Co.,Ltd.). It was found to have an MFR of 72.7 g/10 min (at 250° C., 5 kgf).This value suggests good melt flow rate.

Further, the composition for synthetic resin magnets wasinjection-molded with magnetization, into a cylindrical test piece, 20mm in diameter and 6 mm height. The test piece was examined for magneticenergy product (BH_(max)). The result was 54.91 kJ/m³. This valuesuggests a strong magnetic force.

Comparative Example 1

The same procedure as in Example 1 was repeated to prepare thecomposition for synthetic resin magnets, except that the aliphaticpolyamide was not used and the amount of the nylon-12 was increased to8.5 g. The resulting resin compound was examined for change in torquevalue during melt-mixing in the same way as in Example 1. The resultsare shown in FIG. 1. The sample was also measured for MFR and BH_(max)in the same way as in Example 1.

The sample in Comparative Example 1 did not increase in torque valuesduring melt-mixing as shown in FIG. 1; however, it was poor in melt flowproperties, with an MFR value being 9.48 g/10 min (at 250° C., 5 kgf).In addition, it was also inferior in BH_(max) (51.73 kJ/m³) to thesample in Example 1.

Example 2

A magnetic powder with surface treatment was prepared from the followingcomponents. Sr ferrite: 50.00 kg, “NF110” from Nippon Bengara Kogyo Co.,Ltd. Ba ferrite: 20.55 kg, “DNP-S” from Nippon Bengara Kogyo Co., Ltd.Silane coupling agent: 0.71 kg, “A1100” from Nippon Unicar Co., Ltd.

The thus obtained magnetic powder was mixed with following components byusing a twin-screw mixer. Nylon-6: 12.5 kg, “P 1010” from UbeIndustries. Ltd., Antioxidant: 0.42 kg, “IRGANOX 245” from CibaSpecialty Chemicals K.K., Aliphatic polyamide: 0.42 kg, “PA-30L” fromFuji Kasei Co., Ltd. The resulting mixture was palletized to give thecomposition for synthetic resin magnets pertaining to the first aspectof the present invention.

The thus obtained composition for synthetic resin magnets was measuredfor flow rate (MFR) by using a melt indexer (made by Toyo Seiki Co.,Ltd.). It was found to have an MFR of 156.84 g/10 min (at 270° C., 5kgf). This value suggests good melt flow properties. Further, thecomposition for synthetic resin magnets was injection-molded in amagnetic field into a cylindrical plastic magnet, 9.6 mm in diameter.The plastic magnet was measured for surface magnetic force. A value of80.5 mT was obtained.

Comparative Example 2

The same procedure as in Example 1 was repeated to prepare the resincompound (in pellet form) for plastic magnets, except that the aliphaticpolyamide was not used and the amount of the nylon-6 was increased by0.42 kg to 12.92 kg.

The resulting composition for synthetic resin magnets was measured forMFR in the same way as in Example 2. The measured MFR was 123.99 g/10min (at 270° C., 5 kgf). This result suggests that the sample inComparative Example 2 is inferior in melt flow properties to that inExample 2.

Further, the composition for synthetic resin magnets was made into acylindrical plastic magnet in the same way as in Example 2. The plasticmagnet was measured for surface magnetic force. A value of 789.9 mT wasobtained. This value is lower than that in Example 2.

Example 3

A magnetic powder having an average particle diameter of 50 μm wasprepared by crushing an Nd-based rare earth alloy having a compositionof Nd₁₂Fe₇₈Co₄B₆ in atom wt % (“MQP-B” having an average particlediameter of 50 μm, made by General Motors Inc). The magnetic powder wassurface-treated with a silane coupling agent, “A1100” from Nippon UnicarCo., Ltd. The following components were mixed by using a single-screwmixer. Magnetic powder: 1880 g (mentioned above), Nylon-12 as a resinbinder: 120 g, “P 3012 U” from Ube Industries. Ltd.,N,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]-hydrazine (as ahindered phenol antioxidant): 50 g, “IRGANOX MD 1024” from CibaSpecialty Chemicals K.K.

The resulting mixture was palletized to give the desired resin compound(in pellet form) for plastic magnets. This resin compound was found tohave an MFR of 185 g/10 min (at 250° C., 5 kgf). The composition forsynthetic resin magnets was injection-molded under the followingconditions. Cylinder temperature: 270° C., Mold temperature: 150° C.,Injection pressure: 100 kg/cm², Gate: at one end of the molded piece.Thus there was obtained a formed resin magnet (300 mm long with a crosssection of 3×3 mm) pertaining to the second aspect of the presentinvention. The resulting sample was found to have a high dimensionalaccuracy and a uniform surface magnetic force.

Comparative Example 3

The same procedure as in Example 3 was repeated to prepare thecomposition for synthetic resin magnets, except that the amount of thenylon-12 was increased to 170 g and the hindered phenol antioxidant“IRGANOX MD 1024” was not added. The resulting resin compound was foundto have an MFR of 97 g/10 min (at 250° C., 5 kgf). This value isconsiderably lower than that in Example 3. The resin compound wasinjection-molded into a plastic magnet under the same condition as inExample 3. No satisfactory plastic magnets were obtained due to poormoldability causing short shot.

Comparative Example 4

The same procedure as in Example 3 was repeated to produce a plasticmagnet from the composition for synthetic resin magnets having the samecomposition as in Example 3, except that the mold temperature ininjection molding was reduced to 100° C. No satisfactory plastic magnetswere obtained due to poor moldability causing short shot.

Example 4

The same procedure as in Example 3 was repeated to give the compositionfor synthetic resin magnets, except that theN,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hydrazine (as ahindered phenol antioxidant) was replaced by the same amount (50 g) oftriethyleneglycol bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate,“IRGANOX 245” from Ciba Specialty Chemicals K.K. The resulting resincompound was found to have an MFR of 133 g/10 min (at 250° C., 5 kgf).

This resin compound was molded into plastic magnets pertaining to thesecond aspect of the present invention, under the same conditions as inExample 3. Satisfactory molded products were obtained.

Comparative Example 5

The same procedure as in Example 4 was repeated to produce a plasticmagnet from the composition for synthetic resin magnets having the samecomposition as in Example 3, except that the mold temperature ininjection molding was reduced to 100° C. No satisfactory plastic magnetswere obtained due to poor moldability causing short shot.

1. A composition for synthetic resin magnets which is composed of aresin binder and a magnetic powder mixed and dispersed therein, whereinsaid resin binder comprises a thermoplastic resin as a major constituentand an aliphatic polyamide (excluding that of elastomer type), whereinthe aliphatic polyamide is represented by the formula (1) below:

where R₁ denotes HOOC(CH₂)_(n)COOH (n=7 or 8), C_(m) denotes a diamineresidue chain (m=2-20), C_(n) denotes a dimer acid residue chain(n=20-48), a is an integer of 1-50, b is an integer of 1-50, and x is aninteger of 1 to
 50. 2. The composition for synthetic resin magnets asdefined in claim 1, which contains an antioxidant.
 3. The compositionfor synthetic resin magnets as defined in claim 2, wherein the magneticpowder, major constituent resin, aliphatic polyamide, and antioxidantare mixed in the following ratio. magnetic powder:  80-99 wt %; majorconstituent resin:   1-20 wt %; aliphatic polyamide: 0.1-20 wt %; andantioxidant: 0.1-20 wt %.


4. A formed resin magnet which is formed in a desired shape from thecomposition for synthetic resin magnets as defined in claim
 1. 5. Theformed resin magnet as defined in claim 4, which is a magnet roller usedfor development in electrophotography process.